We have now established that the myoblast line in culture can differentiate in a chemically defined medium. Whole fetal calf serum (F.C.S.) is required to maintain through serial passage the competence of the myoblast line to differentiate; it is required only in trace amounts for the process of differentiation of myoblasts into myofibers. We have further discovered that mononucleated myoblasts become "committed" to differentiate during culture in medium low in F.C.S., and proceed to do so even when cultural conditions are modified in order to favor growth over differentiation.